Field of the Invention
A process for the production of composites containing reinforcing fiber and a fluoropolymer.
Polymeric composites consisting of a matrix resin (including fluoropolymers) and a reinforcing fiber is an area well known in the art. By adding the fiber to the matrix resin, certain properties of the resin alone are often improved. These properties can include creep, tensile strength and modulus, flexural strength and modulus. Generally speaking, the reinforcing fiber chosen has a higher tensile modulus and strength than the polymer alone. When a fluoropolymer is used as the matrix resin, as described herein, the resulting composite often has many of the attributes of fluoropolymers such as high temperature resistance and chemical resistance, which make such composites useful in part, for example, for the chemical processing industry and semiconductor industry. It is the object of this invention to provide a method for the production of such fluoropolymer composites that exhibit improved properties.
U.S. Pat. No. 4,163,742, which is incorporated herein by reference, describes a process for making tetrafluoroethylene copolymer-graphite fiber composite by mixing the polymer and fiber together in a slurry and then removing the water to obtain an intimate mix of fiber and polymer. It is also known that fluoropolymers have very low surface energy and are difficult to wet in an aqueous media for mixing, as well as wet the reinforcing fiber in the molten state to form a composite. Thus, very fine particles are used to enhance the intimacy of the fiber resin mixture. This complicates mixing by requiring extensive use of organic solvents or emulsifying agents to assist wetting. Further, this complicates consolidation by creating a mix with a very high bulk factor with dry mix typically being ten times the volume of a consolidated composite.
As taught in U.S. Pat. No. 4,163,742, pressing the mix at 5000 to 6000 psi was required to cold compact it, allowing a stepwise filling of the mold. In this step the mix is debulked as air is removed; the resin cold flows to form a stable block. Subsequent heating coalesces the block into a solid. This technique is common with Teflon.RTM. materials. Further, experience showed that heating the resin fiber mix in a mold without cold compaction caused the resin to flow and leak from the mold before enough pressure was built up to remove the air and consolidate the block. However, cold compaction causes significant fiber breakage and requires deep draft molds and very high pressures. It is highly desirable to find a method for producing consolidated composites of intimately co-dispersed, finely divided fluoropolymer and other thermoplastic resins with reinforcing fibers without the need to severely damage the reinforcement in the process.